A parasitic relationship is one in which one organism, the parasite, lives off of another organism, the host, harming it and possibly causing death. The parasite lives on or in the body of the host.

A few examples of parasites are tapeworms, fleas, and barnacles. Tapeworms are segmented flatworms that attach themselves to the insides of the intestines of animals such as cows, pigs, and humans. They get food by eating the host's partly digested food, depriving the host of nutrients. Fleas harm their hosts, such as dogs, by biting their skin, sucking their blood, and causing them to itch. The fleas, in turn, get food and a warm home. Barnacles, which live on the bodies of whales, do not seriously harm their hosts, but they do itch and are annoying.

Usually, although parasites harm their hosts, it is in the parasite's best interest not to kill the host, because it relies on the host's body and body functions, such as digestion or blood circulation, to live.

Some parasitic animals attack plants. Aphids are insects that eat the sap from the plants on which they live. Parasitic plants and fungi can attack animals. A fungus causes lumpy jaw, a disease that injures the jaws of cattle and hogs. There are also parasitic plants and fungi that attack other plants and fungi. A parasitic fungus causes wheat rust and the downy mildew fungus attacks fruit and vegetables. Some scientists say that one-celled bacteria and viruses that live in animals and harm them, such as those that cause the common cold, are parasites as well. However, they are still considered different from other parasites. Many types of parasites carry and transmit disease. Lyme disease is trasmitted by deer ticks.

A parasite and its host evolve together. The parasite adapts to its environment by living in and using the host in ways that harm it. Hosts also develop ways of getting rid of or protecting themselves from parasites. For example, they can scratch away ticks. Some hosts also build a symbiotic relationship with another organism that helps to get rid of the parasite. Ladybugs live on plants, eating the aphids and benefiting by getting food, while the plant benefits by being rid of the aphids.